1. Field
This invention is directed to recording media with reduced unintentional erasure. In particular, the invention is directed to methods, systems and components that allow for reduced sidetrack erasure (STE).
2. Related Arts
Magnetic media are widely used in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as “longitudinal” or “perpendicular” depending upon the orientation of the magnetic domains of the grains of magnetic material.
Perpendicular recording media (recording media with a perpendicular anisotropy (Hk) in the magnetic layer and magnetization forming in a direction perpendicular to the surface of the magnetic layers) have been found to be superior to longitudinal media in achieving very high bit densities without experiencing the thermal stability limit associated with the latter. In perpendicular magnetic recording media, residual magnetization is formed in a direction (“easy axis”) perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high to ultra-high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
Typically, perpendicular recording media are fabricated with polycrystalline CoCr or CoPt-oxide containing films. Co-rich areas in the polycrystalline film are ferromagnetic while Cr or oxide rich areas in the film are non-magnetic. Magnetic interaction between adjacent ferromagnetic domains is attenuated by nonmagnetic areas in between.
A traditional, perpendicular magnetic medium utilizes multiple layer interposition including a relatively thick (as compared with the magnetic recording layer), “soft” magnetically permeable underlayer (“SUL”), i.e., a magnetic layer having a relatively low coercivity below about 1 kOe, such as of a NiFe alloy (Permalloy), (or a material that is easily magnetized and demagnetized), between a non-magnetic interlayer, e.g., of glass, aluminum (Al) or an Al-based alloy, and a magnetically “hard” recording layer having relatively high coercivity, typically about 3-8 kOe, e.g., of a cobalt-based alloy (e.g., a Co—Cr alloy such as CoCrPtB) (or a material that neither magnetizes nor demagnetizes easily) having perpendicular anisotropy. The magnetically soft underlayer serves as a magnetic flux return path for the field from the write pole to the return pole of the recording head. See U.S. Publication No. 20070287031 and U.S. Pat. No. 6,914,749.
A typical conventional perpendicular recording system with a perpendicularly oriented magnetic medium and a magnetic transducer head commonly comprises a non-magnetic substrate, an optional adhesion layer, a relatively thick magnetically soft underlayer (SUL), an interlayer stack comprising at least one non-magnetic interlayer, sometimes referred to as an “intermediate” layer, and at least one relatively thin magnetically hard perpendicular recording layer with its magnetic easy axis perpendicular to the film plane. The interlayer stack preferably includes at least one interlayer of an hcp (hexagonally close packed) material adjacent the magnetically hard perpendicular recording layer and an optional seed layer adjacent the magnetically soft underlayer (SUL), preferably comprising at least one of an amorphous material and an fcc material.
A recording system further comprises main (writing) and auxiliary poles of the magnetic transducer head. A relatively thin interlayer, comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the magnetically soft underlayer and the at least one magnetically hard recording layer; and (2) promote desired microstructural and magnetic properties of the at least one magnetically hard recording layer.
Magnetic flux (φ), emanates from the main writing pole of a magnetic transducer head, enters and passes through the at least one vertically oriented, magnetically hard recording layer in the region below main pole, enters and travels within the SUL for a distance, and then exits therefrom and passes through the at least one perpendicular hard magnetic recording layer in the region below the auxiliary pole of the transducer head.
Completing the layer stack of the medium is a protective overcoat layer, such as of a diamond-like carbon (DLC), formed over magnetically hard layer, and preferably a lubricant topcoat layer, such as of a perfluoropolyether (PFPE) material, formed over the protective overcoat layer. The protective overcoat protects the magnetic recording layer from corrosion and reduces frictional forces between the disc and a read/write head. In addition, a thin layer of lubricant may be applied to the surface of the protective overcoat to enhance the tribological performance of the head-disc interface by reducing friction and wear of the protective overcoat.
The substrate is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having a Ni—P plating layer on the deposition surface thereof, or alternatively, the substrate is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials. The optional adhesion layer, if present on the substrate surface, typically comprises a less than about 200 Angstrom (Å) thick layer of a metal or a metal alloy material such as Ti, a Ti-based alloy, Ta, a Ta-based alloy, Cr, or a Cr-based alloy. The relatively thick soft magnetic underlayer is typically comprised of an about 50 to about 300 nm thick layer of a soft magnetic material such as Ni, Co, Fe, an Fe-containing alloy such as NiFe (Permalloy), FeN, FeSiAl, FeSiAlN, a Co-containing alloy such as CoZr, CoZrCr, CoZrNb, or a Co—Fe-containing alloy such as CoFeZrNb, CoFe, FeCoB, and FeCoC. The relatively thin interlayer stack typically comprises an about 50 to about 300 Å thick layer or layers of non-magnetic material(s). The interlayer stack includes at least one interlayer of an hcp material, such as Ru, TiCr, Ru/CoCr 37 Pt 6, RuCr/CoCrPt, etc., adjacent the magnetically hard perpendicular recording layer. When present, a seed layer adjacent the magnetically soft underlayer (SUL) may typically include a less than about 100 Å thick layer of an FCC material, such as an alloy of Cu, Ag, Pt, or Au, or an amorphous or fine-grained material, such as Ta, TaW, CrTa, Ti, TiN, TiW, or TiCr. The at least one magnetically hard perpendicular recording layer is typically comprised of an about 10 to about 25 nm thick layer(s) of Co-based alloy(s) including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, W, Cr, Ru, Ti, Si, O, V, Nb, Ge, B, and Pd.
Of the conventional media types described above, longitudinal media are more developed than perpendicular media and have been utilized for several decades in the computer industry. During this interval, components and sub-systems, such as transducer heads, channels, and media, have been repeatedly optimized in order to operate efficiently within computer environments. However, it is a common current belief that longitudinal recording is reaching the end of its lifetime as an industry standard in computer applications owing to physical limits, which effectively prevent further increases in areal recording density.
Perpendicular media, on the other hand, are expected to replace longitudinal media in computer-related recording applications and continue the movement toward ever-increasing areal recording densities far beyond the capability of longitudinal media. However, perpendicular media and recording technology are less well developed than all facets of longitudinal media and recording technology. Specifically, each individual component of perpendicular magnetic recording technology, including transducer heads, media, and recording channels, is less developed and optimized than the corresponding component of longitudinal recording technology. Consequently, the benefits observed with perpendicular media and systems vis-à-vis the prior art, i.e., longitudinal media and systems, are yet to be fully realized.
One of the dominant issues facing perpendicular recording is unintentional erasure, particularly sidetrack erasure (STE). FIG. 1 shows a prior art perpendicular recording system 100 illustrating one of the mechanisms for STE. Write head 105 emits a magnetic field (shown by the downward arrows) through overcoat 140 and into hard recording layer 130, where the magnetic field causes data to be written and saved. The magnetic field then passes through interlayer 120 into SUL 110. For a single layer SUL 110, the SUL is typically aligned in the cross track direction. There are two major problems associated with such SULs: 1) due to geometric limitations, the ground state, i.e. non-writing state, of the SUL prefers to have magnetic domains 150, thus leading to large noise from the SUL; and 2) (as shown in FIG. 1) during dynamic writing, the magnetization in the SUL will cause a concentration of charge 160 (a charge density) in the SUL that provide an additional field promoting erasure of the media.
FIG. 2 shows a prior art perpendicular recording system 200, similar to system 100 except that SUL 210 has two layers 212 and 214 of equal thickness, which are used to attempt to overcome the STE. Layer 212 is magnetized in one direction while layer 214 is magnetized in the opposite direction. In the ground state, dual layer SUL prefers to be domain free. Dynamic reversal of the dual layer SUL generally will cause significantly less field erasure as compared to the single layer SUL case (FIG. 1). However, the impact of the demagnetization field for the same rotation of magnetization in each layer of the SUL is different; layer 212 has more impact than lower layer 214. In addition, due to recording geometry, layer 212 also has a larger rotation angle. Therefore, the SUL still responds asymmetrically during a dynamic recording process (e.g., writing of data), such that a residue charge continues to exist in the SUL, and which may contribute to sidetrack erasure of data recorded on the hard recording layer 230.
In view of the foregoing, there exists a need for improved perpendicular media and system technology. In particular, media with reduced STE would advance the adoption of perpendicular media technology.